This invention relates to an image reading apparatus.
Conventionally known image reading apparatus are of flatbed (FB) type and of auto document feeder (ADF) type. The reading image apparatus of FB type reads a document by conveying in a sub-scanning direction a line sensor that is able to read an image in a main scanning direction. The reading image apparatus of ADF type reads a document by fixing a line sensor and conveying a document in a sub-scanning direction. In these types of image reading apparatus, the document or the line sensor is conveyed so that a relative position between the document and the line sensor is changed at a constant speed. During the constant speed conveyance, the line sensor executes reading operation per predetermined time period and generates line image data which is the read image data per line. Each of the generated line image data is combined with one another so as to generate the read image data representing a read image of the document.
It is also known that, in the above-described image reading apparatus, the line image data is temporarily accumulated in a buffer and then transferred to the downstream. In the case that the buffer becomes or is likely to become full due to process delay or for other reasons in the downstream, document reading based on the constant speed conveyance is temporarily interrupted. Then, the position where the document reading has been interrupted is stored and held. When a free space is created in the buffer, the document reading based on the constant speed conveyance is restarted from the interrupted position. In order to accurately specify the interrupted position, a configuration is known which provides a line start signal to the line sensor in synchronization with an encoder signal of an encoder which outputs the encoder signal in accordance with the move of the line sensor.